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Preferred Instruments

Preferred Instruments manufactures microprocessor based controllers, instruments, and electric actuators for combustion and process applications. Preferred Instruments also manufactures oil tank gauging and leak detection equipment. A major component of the Instrument group sales is from control systems integration and personal computer based Supervisory Control and Data Acquisition systems. Preferred Utilities' industrial factory is located in Danbury, CT

In-Situ Oxygen Sensor

Preferred Instruments engineers and manufactures boiler control systems for commercial, industrial, and institutional facilities. The Model ZP, In-Situ Oxygen Sensor, provides reliable long life flue gas oxygen measurements.


The Preferred ZP series zirconium oxide oxygen analyzer works in conjunction with Preferred PCC-III and DCS-III loop controllers, and the BurnerMate Universal boiler control system.

Zirconium oxide in-situ oxygen analyzers are designed to be mounted permanently in the boiler stack. The zirconium oxide cell, when heated to 800 deg. C, produces a mV signal proportional to measured oxygen on a wet basis. The Preferred controller provides all probe control functions including temperature monitoring, cell temperature maintenance, monitoring, and calibration. This system is much more reliable than extractive sampling (EGA) systems that are sensitive to moisture, and rely on chemical cells that are designed to deteriorate with time and usage.

The Preferred ZP probe can be applied as simply an oxygen analyzer when combined with a pre-programmed PCC-III loop controller. It can also be used to save fuel as an oxygen trim controller when combined with Preferred LTA systems, UtilitySaver systems, BurnerMate TS systems, and the BurnerMate Universal.


Preferred Instruments engineers and manufactures boiler control systems for commercial, industrial, and institutional facilities. Preferred’s boiler control systems include combustion, feedwater (drum level), draft & flue gas recirculation control functions. Preferred’s integrated control systems provide a full scope control package that assures safe and efficient control with undivided system integration responsibility.

Detector: The detector consists of a zirconium oxide cell, a ceramic heater with thermocouple, terminals for connecting to the controller unit, a flange for connection to the probe, opening to accept reference (ambient) air and a connection for calibration gas. The detector works on a principle that when heated to 800° C (1472° F), the cell generates an electrical signal directly related to the Oxygen concentration of the flue gas. Flue gases are passed through a filter to prevent dust and dirt from contaminating the cell. Calibration gas can be injected into the space behind the ceramic filter to allow on-line calibration without removal from the stack.

Probe: The probe is a stainless steel assembly that mounts on a 3″ 125 lb flange (flat face) located on the flue gas duct or stack. The probe protrudes into the flue gas stream and directs boiler flue gases from the middle third of the flue gas stream to the detector. The assembly’s design provides for the removal of the detector for service or replacement without the need for removal of the entire probe.



  • Reliable, Long Life Flue Gas Oxygen Measurement
  • Separate Field Mounted Transmitter Not Required
  • Simple Automated Calibration
  • Integral Oxygen Trim and Boiler Efficiency Control Logic
  • Instrument Air Is Not Required


Gas Measured: Oxygen In Flue Gases

Sensing Method: In-Situ Field-Replaceable Zirconim Detector, and Reliable Ceramic Heater

Flue Gas Temperature: 0 to 1150° F

Measuring Range: 0 to 10% Standard, 0 to 21% Field Configurable

Accuracy: +/- 1% of Reading or 0.1% O2 (Calibration Gas Dependent)

Response Time: Initial 0.1 sec / 90% 7 sec

Power Supply: 120 VAC, 60 Hz

Power Consumption: 82 VA Total (ZP and PCC-III)

Connecting Cable: 190130. Combined Signal and Power Cable. Up to 500 ft.

Indicating Electronics

Instrument: PCC-III Controller, Up To Two Probes Per PCC-III

Oxygen Trim Options: Jackshaft, Parallel Positioning or Fully Metered Combustion Applications

Boiler Efficiency: Logic Included, Optional Flue Gas Temperature T/C Required

Oxygen Signal: 4-20 mADC, Linear, Modbus RS485. (Blockware Dependent)

Ambient: 32-130º F

Case: Weather-Proof Front Panel

Power Supply: 115 VAC, 60 Hz

Display: 4.5 Digit LED Numeric Display, 6 Status LEDs

PCC-III “Z” Option Board

Isolated Inputs:

  • Probe mV Input
  • Probe Heater Type R T/C Input
  • Spare Type J T/C Input

Output:  Fused 120 VAC Heater Supply


Application: Natural Gas, Fuel Oils

Sample gas: 0 to 1150° F

Flanges: ANSI 125 #, 4 Bolt, 3 Inch Flange

Probe Lengths: 20, 30, 45, 65 or 90 Inch

Wetted Parts: 304 Stainless Steel, Alumina, Quartz, Zirconim, Platinum

Weight: Probe of 20 Inch Length Approx. 18 lb.

Life of Zirconia Cell: Typically 3-4 Years (1 Year Warranty)

Construction:  NEMA 12


  • 190130 Connecting Cable, (Requires ½” Conduit Minimum)
  • 90157 Flame Arrester
  • 90159 Rain Shield for Outdoor installation
  • EPA CEM Auto Calibration Package


Provide a boiler breeching mounted in-situ, zirconium oxide Oxygen analyzer for each boiler. Extractive or “Wet Cell” type Oxygen analyzers are not acceptable.  The probe shall be of a suitable length to sense the Oxygen level in the middle ⅓ of the breeching.  All wetted parts shall be stainless steel. The Oxygen analyzer shall include a digital controller that performs continuous self-diagnostics with diagnostic codes for at least 10 common faults. The system shall automatically send the trim actuator to the ‘null’ position and trigger the alarm dry contacts in the event of an Oxygen analyzer fault. The detector shall be field replaceable without removing the probe from the stack and shall not require special tools. The analyzer shall automatically perform periodic detector cell impedance tests to be used by the operator as an indication of calibration shift. Analyzer calibration shall be pushbutton semi-automatic (no trim pots) with English language prompts and diagnostic messages.  Analyzer output shall be field selectable as 0-10% or 0-21% without field re-calibration.